Treating sewage



Oct. 31, 1939. 1G. BEVAN TREATING SEWAGE Filed Dec.

kwamen@ 2,111,851

JUNITED -s'rA'rss PATENT or-Flcs anus-1 l f l TBEATIN G SEWAGE John G.Bevan, New York, N. YY., assignor to Guggenheim Brothers, New York, N.Y., a oopartnership Application member es, V 1934, sum No., 159,451

z clim. (c1. 21o- 2) This invention relates to sewage treatment and ormore compounds are employed any or all may has for an object theprovision ofv an improved be added as solids and any or all may be addedprocess for treating sewage and similar waste' in solution. Thus, forexample, I may iirst add liquors. More particularly, theinventioncontema dry soluble salt of aluminum or a solution 5 plates theprovision of an improved process for of an aluminum vcompound to thesewage and 5 removing putrescible matter from sewage and later add a drysoluble salt of iron or a solution similarwaste liquors. The inventionfurther convof an iron compound. I prefer to add iron comtemplates theprovision of a process for treating pounds in solution, as solidparticles of such comsewage involving the use of sludge obtaineddurpounds become coated with sewage solids readily ing the course of theprocess in conjunction with and solutionand dispersion are hindered orprel0 an iron compound or a compound of a trivalent vented. Aluminumcompounds may be employed metal for coagulation of suspended matterconadvantageously either in solid form or in solutained in the sewage.The process of the invention form. Particles of aluminum compounds dotion may also include the use of an alkaline not acquire coatings ofSewage solids when added 16 agent such as lime when necessary, foradjusting to Sewage. I have also discovered that coagul5 rthe pI-I valueof the sewage in order'to provide llation Of Suspended matte!` containedin Sewage for improved settling and improved filtration. may be promotedby adding t0 the Sewage a The invention can be employed with particularSludge produced lu a DIOCeSS ,involving the use advantage in the iirststage of the sewage ltreatof a metal compound aS a coagulatng agent, 2oing process described in the United stai-,e5 patent after treatment ofthe sludge with an acid capable to Gleason and Loonam,'No. 1,886,267 ofNovem- 0f reactingl With the metal of the coagulating 20 ber l, 1932.compound to produce a soluble compound of the In ordinary raw sewagesand similar waste metalliquors, putrescible matter, consisting ofcarbon, The acid-treated Sludge may be employed. in hydrogen, sulphur,etc. in various combinations, elly-Suitable manner. Thus, for example. ap0r- 25 is present in twoforms, namely; (l) insoluble vtion of theSewage to be treated may be subjected (suspended matter), (2) soluble.In any sewage to the action of the metal compound and another treatingoperation, it is'customary to remove the Portion may be subjected to theaction of acidcoarser matter by screening. After screening, the treatedSludge from Such u process, 0r the acidconcentration of the insolubleputrescible matter treated Sludge may be employed in conjunction 30 Vremaining in the sewage is relatively low, for ex- Wltll the metalCompound in the treatment of' a ample, an average municipal sewagecontains Single batch 01 lloW of Sewage. When the acidaround 150 to 200parts per million of dispersed treated Sludge iS employed in conjunctionwith or nnely divided solid matter largely 1n 011oida1 the metalcompound in the treatment of a single Y suspension, This finely dividedputrescible matbatch or ilow of sewage, the acidi-treated sludge terincludes simple and complex compounds reand the metal compound may beemployed at the 35 sulting from the breaking down of protein same timeor alternately. Thus, for example, the The present inventioncontemplates certain m- Sewage may be treated fol a Suitable period ofprovements in the removal of putrescible matter time With the meta-lCOmDOllud- During this De- 40 from sewage coagulation, ilocculation orentrain- IOd Sludge may be WtlldfdWncoutnuouSly and 40 ment in asettleable sludge. I have discovered treated With acldwhen a Suitableamount of that the removal of putrescible matter from sewacid-treatedSludge has accumulated, treatment age is promoted 4loy aerating thesewage4 in the with the metal compound may b e discontinued presence ofan added metal Compound such, for and-treatment with the acid-treatedsludge comexample, asacompound of iron or aluminum and menced andContinued aslong as an adequate 45 a quantity of sludge produced orrecovered dur- Supply is avalable- Ilf the amd-treated Sludge ing thecourse of the process. I prefer to employ and the metal Compound areemployed at the one or more soluble compounds of iron 0r alumisame time,a portion of the sludge recovered durnum for addition to the sewagebeing aerated. lug the proceSS may be Withdrawn and subjected -I mayemploy, for example, ferrous sulphate, ferto the action of acidcontinuously to maintain 50 ric sulphate, 'ferrie chloride, aluminumchloride, an adequate Supply 0f acid-treated Sludge. aluminum sulphateor manganese sulphate, or The sludge may be treated with commercial anydesired mixtures-of these compounds. In or relatively pure acid or asolution of a metal employing metal compounds, I may add them tocompound containing excess acid, for example, a

the sewage as solids or in solution. When two solution containing ironsulphate or iron sulphate 55 and aluminum sulphate together withfreesulphuric acid, may be employed for treating the sludge. In this mannera complete coagulating agent containing new or original metal compoundand regenerated metal compound may be produced.

In preparing acid-treated sludge, it is advisable to mix the acid withthe sludge under oxidizing conditions, as, for example, by means of airor in the presence of an oxidizing agent, in ordei to maintain the metalcompound in oxidized condition and to prevent re-solution ofprecipitated organic compounds. It is also advisable to malntainoxidizing conditions within the body of acidtreated sludge from its timeof production until its time of use. This may be accomplished byintroducing an oxidizing gas such as air or an oxidizing agent such assodium nitrate into the body of acid-treated sludge.

When the acid-treated sludge and the metal compound are employed fortreating a single batch or flow of sewage, they may be added together orseparately, and when added separately either may be added first and theseparate additions may be spaced at any suitable intervals.

An untreated portion of the recovered sludge (a portion not treated withacid) may be returned to the sewage undergoing treatment or not, asdesired.

In a preferred complete process of the invention, sewage is aerated inthe presence of a metal compound and untreated sludge and the untreatedsludge is added,to the sewage at the commencement of the aerationtreatment or shortly after the raw sewage enters the aeration vessel orbasin. The metal compound may be added to the sewage at any desiredstage. of the aeration treatment. When ferric sulphate is employed, itis preferably added to the sewage during the course of its aerationwithin about two to fifteen minutes before completion of the aerationtreatment. Aluminum sulphate and aluminum chloride also are preferablyadded within about two to fifteen minutes before completion of theaeration treatment. Ferrous sulphate is preferably added to the sewagewithin about ten to thirty minutes before the end of the aerationtreatment.

Acid-treated sludge may be added at the same time as the iron oraluminum compound or addition of the iron or aluminum compound may bediscontinued while the acid-treated sludge is being added. When themetal compound employed is iron or aluminum, the sludge is preferablytreated with sulphuric acid in amount about sufcient to form. ferriesulphate with the iron in the sludge or aluminum sulphate with thealuminum in the sludge. Excess acid may be employed, if desired. Thetime of addition of acidtreated sludge containing iron or aluminumsulphate may correspond substantially with the time of addition of ironor aluminum sulphate. Also the amount of metal compound added to sewageis preferably maintained substantially the same whether added as a newor original compound, as a regenerated compound in acidtreated sludge,or as a mixture of new or original compound and regenerated compound inacidtreated sludge.

The acid employed for treating the sludge need not be an acid of whichthe metal compound added to the sewage is a salt. For example, a metalchloride may be added to the sewage and the sludge may be treated withsulphuric acid to produce a metal sulphate. or an insoluble compoundsuch as a metal oxide may be added to the sewage and the sludge may betreated with any suitable acid capabl of forming a soluble' compound ofthe metal.

Good results may be obtained if the period of aeration following theaddition of a compound such as ferric sulphate or aluminum sulphateseparately or in acid-treated sludge is just Sullicient to permitthorough dissemination of the compound. The period of aeration may bevaried considerably. Good results may be obtained if aeration is carriedout for a period of about one hour or less, and little or no additionaladvantage may be gained by continuing aeration for a period of timelonger than about one hour.

If the pH of the sewage is 7 or lower near the end of the aerationperiod and it is desired to increase the pH to a value above 7, anamount of an alkaline agent such as lime suicient to increase the pH tothe desired value may be added prior to the completion of the aerationtreatment. Usually, it will not be necessary or desirable to adjustthepH of the sewage by the addition of an alkaline agent, as I have foundthat highly satisfactory precipitation, settling and clarication can beobtained even when the pH is considerably below 7. If the addition of analkaline agent is desired or required for any purpose, it is preferablyadded after the soluble metal compound or acid-treated sludge or bothhave been added and thoroughly disseminated throughout the sewage.Following the aeration treatment, the sewage is introduced into aclarier underneath a layer or blanket of-settling sludge so that theliquor flows upwardly through the sludge blanket and overflows from theclariner with a practically negligible amount of suspended matter.

In carrying out the process of the invention in its preferred andcomplete form, the raw sewage, after appropriate screening is mixed withuntreated returned sludge in an aeration vessel and aeration of themixture is commenced. The aeration vessel employed is preferably of thetype which permits a continuous ow of the mixture undergoing treatmentfrom an entrance end to a discharge end, and the rate of ow ispreferably controlled to provide a period of about one hour between thetime of entry and the time of discharge of the sewage. Aeration of thesewage is preferably continued throughout its entire period of detentionin the aeration vessel.

As the mixture undergoing treatment is flowing through the aerationvessel, a soluble iron compound, preferably in the form of a ferricsulphate solution of 50 grams per liter concentration and a quantity ofacid-treated sludge are added to the mixture at a point corresponding toan interval of' about 2 to 15 minutes before the time of discharge ofthe mixture from the vessel. Prompt diffusion of the solution and theacid-treated sewage throughout the mixture is produced by the agitationresulting from aeration. The oxygen in the air keeps the iron in theferrie condition. When a ferrous salt such as ferrous sulphate isemployed, the oxygen o1 the air converts the iron to the ferriecondition.

'Ihe incoming raw sewage ordinarily has a pH of around 'l to 7.5. In thecoagulating treatment, the trivalent metal ions are precipitated,

probably in the form of ferrie hydroxide as well as in the form ofsimple and complex organic ferric compounds. 'Ihe sulphate ions of theadded ferric sulphate and any excess acid in the acid-treated sewagetend to lower the pH of the amasar 3 sewage to' a value below 7, butthis tendency is largely counteracted by the eiIects of the aeration andthe returned sludge, Vand the sewage at the conclusion of the ferricsulphate treat- 5 ment may have a pH equivalentto that of theincomlngraw sewage. Substantially all the iron added as ferric sulphate,either as a solution of the salt or in the acid-treated sludge. isprecipitated, and it appears that this precipitated iron together withthe iron returned with the untreated sludge forms a blanket of ferrichydroxide which acts as a collector of suspended matter. The returneduntreated and acidtreated sludge and the aeration treatment also aid ineffecting precipitation of the suspended matter. Also, aeration of thesewage causes oxidation of a considerable amount of the dis- L solvedputrescible matter.

The total amount of iron added to the sewage (as an iron salt orsolution-and in the acidtreated sludge) depends to some extent at least,upon the amount of putrescible matter in the sewage. In the treatment ofthe average municipal sewages, a total of 5 to 25 parts' per mil- 26lion of added iron produces satisfactory coagulation and precipitation'of the putrescible matter, and, generally, a total of less than 20parts per million of iron produces satisfactory results. In thetreatment of industrial waste liquors such,` 30 for example, as dairyplant and packing plant waste liquors., a total of 20 to 50 parts permillion of 'iron may be required to produce satisfactory coagulation andprecipitation. Satisfactory aeration is obtained with from 0.01 to 0.1cubic foot of air per gallon of sewage treated. It appears that theoxygen of the air employed for aeration functions chiefly to oxidizeputrescible matter and is not absorbed by the sludge. This is indicatedby rapid elimination of dissolved 4o putrescible matter and the presenceof dissolved oxygen in the clarifier overflow.

If adjustment of the pH is desirable following the treatment with ferriesulphate, the sewage may be treated inthe presence of an alkaline agent,such as lime, in amount sufficient to give a pH of the desired value.Adjustment of the pH to a value above '7 may sometimes improve settlingand illtration. Lime is preferable to other alkaline agents for thisoperation because it gives a denser floc and one which settles verylrapidly, and because it isinexpensive. Lime is preferably added to thesewage as milk of lime in 50 grams per liter concentration. Wheredissolved putrescible matter is to be subsequently removed from thesewage liquor by a base exchange (zeolite) treatment, it is preferableto carefully regulate the amount of lime added to the sewage to give apI-I of about 7.2 to 7.6.

Following the treatment with lime, the sewage passes to aclarier, into,which it is introduced near the bottom, passing upward throughl ablanket-of settling sludge. Claried liquor overflows and settledI sludgeis removed from the bottom of the clarier, preferably being raked a5 orotherwise mechanically worked along the bottom of the clarifier towardthe sludge discharge outlet. The supernatant liquor is substantiallyfree from suspended matter and may advantageously be subjected to a baseexchange' treat- 7u ment as described in the aforementioned patent toGleason and Loonam.

The sludgefrom the bottom of the clariiler may be conducted in part to'a thickener and in part to the aeration vessel. Clarified liquor over-75 flows and settled sludge is removed from the bottom of the thickener,The clarifier and thlckener efiluents may be' combined for appropriate'disposal or for subsequent treatment, but the thickener emuent ispreferably introduced into the aeration vessel at the entrance end forretreat- 5 ment. The sludge from the thickener, containing about 3% to10% solids, may be conducted in part to the aeration vessel, in'part toan acidtreatment vessel or mixer and in partk to a 4suc-V tion illterwhere its moisture content is reduced. 10 'I'he illter eliluent orfiltrate is preferably returned to the aeration vessel for re-treatment.

All of the untreated sludge returned to the aeration vessel may beobtained from the clarifier, all may be obtained from the thickener, ora 15 portion may be obtained from the thickener and a portion from theclarier. About 20% to 75%, more or less, of the sludge is returned inun-v treated farm to the aeration vessel. The amount Vof untreatedsludge returned may vary con 20 Ssiderably.- Ordinarily, sludge returnmay be controlled advantageously to establish in the sewage-sludgemixture undergoing treatment in the aeration vessel an amount ofsuspended matter equal to about two to eight times the amount of 25suspended matter in the incoming raw sewage.

It is myV preferred practiceto completely destroy-the organic matter inthe filter. cake by incineration. A rotary kiln incinerator consistingv'of two sections, namely, a drying section and a.' 30

burning section is admirably adapted for the purpose. In both the dryerand incinerator sec- -tions of the unit, the air travels concurrentlywith the solids and gases. The dryer section is operated at atemperature which does not cause 35 n.

value of the sludge is such that in a large scale operation noextraneous fuel is required for incineration.

The lncinerated residue is `preferably treated with sulphuric acidforthe regeneration of the ferrie sulphate which is again used in theprocess.

The single figure of the accompanying drawing is a diagrammaticfiowsheet of a system adapted for carrying out the process of thelnvention. g

The raw sewage is first passed through a 55 coarse screen and thenthrough a rie screen (not shown in the drawing)` to remove coarse andheavy solid matter. The screened sewage. is treated according to theprocess of the invention.

In the apparatus illustrated in the drawing, 60

aeration of the sewage is carried out in a long tank or basin one endportion of which receives the incoming sewage and the other end portionof which discharges into a clarler. Untreated returned sludge is addedto the raw sewage as it 55 .enters the aeration basin and thoroughmixing of the sludge and sewage is accomplished by air agitation. In,this apparatus, the rateof ow of the sewage through the aeration basinis such as to give a detention period of .about one hour. Ferriesulphate solution and acid-treated sludge are preferably added'to thesewage in the aeration basin at a point corresponding to an interval ofabout 2 to 15 minutes before the time of discharge of the sewage intothe clarier, but pro- '15 vision is made for the addition of the ferriesulphate and acid-treated sludge at a number of points between theentrance end and the discharge end of the basin. The ferrie sulphate andacid-treated sludge may be added to the sewage in the aeration vessel atone or more points during the course of iiow of the sewage through theaeration vessel. If the addition of an alkaline agent is required ordesired for adjustment of the pH of the sewage, milk of lime is addedbetween the point or points of addition of the ferrie sulphate andacid-treated sludge and the discharge end of the aeration vessel andlspaced sufficiently from the point or points of addition of the ferricsulphate and acid-treated sludge to permit Athorough dissemination oftheferric sulphate and acid-treated sludge in the sewage before thelime isadded. A common air diffuser' extends throughout the length of theaeration vessel and provides a source of air for aeration and agitation.The air diffuser may be of any suitable construction.

'Ihe clarier illustrated in the drawing comprises a tank having aperipheral charging well 5 (extending approximately around one-third iof the circumference of the tank) and discharging into the bottom of thetank. Clarified liquor overiiows in a launder opposite the charging welland extending approximately around onethird of the upper circumferenceof the tank. A raking mechanism l moves sludge settling on the bottom ofthe tank towards the centrally positioned sludge discharge outlet. It isto be understood that other forms and types of clarifier may be used inpracticing the invention. I have found that the introduction of thesewage near the bottom of a settling chamber beneath a settling mass ofsludge therein materially aids in the production of a supernatant liquorsubstantially free from suspended matter.

Sludge from the clarifier is pumped by means of a pump I0 to a thickenerhaving a sludge raking mechanism 8 and an overilow launder 9, or in partto the thickener and in part to the entrance end of the aeration vessel.The sludge pump I0 communicates with a conduit II which leads to asludge return conduit I2. A by-pass conduit I 3 is provided forconducting sludge from the pump I0 and conduit Il'to the thickener.Suitable -vvalves are provided in the conduits Il and I 3 to control anddistribute the ow of sludge therethrough. The clarier eiiluent may besubjected to further treatment, as, for example, by means of zeoliteiilters as described in the aforementioned patent to Gleason and Loonam.The thickener eiiiuent is preferably conducted through a conduit 22,having a suitable pump included therein and communicating with theentrance end portion of the aeration vessel, to the aeration vessel forre-treatment.

A portion of the sludge from the thickener mayl be conducted by means ofa pump I4 and conduit I5 to the sludge return conduit I2 leading to theentrance end of the aeration vessel. A by-pass conduit II is providedfor conducting sludge from the thickener to the acid-mixing tank I 8.The mixing tank is provided with an agitator and 'an' inlet forintroducing air under pressure. The remainder of the sludge from thethickener is conducted through the conduit I6 to a suction lter.Suitable valves are provided in the conduits I5, I6 and I'I to controland distribute the now of sludge therethrough. The filter-cake isconveyed to the dryer and incinerator. The lilter eiiiuent or filtrateis conducted to the entrance end portion of the aeration through theconduit 22 and a conduit 23 which communicates with the lter and theconduit 22. Suitable valves arel provided in the conduits 22 and 23 tocontrol the ow of liquids therethrough.

Sludge is conducted to the acid mixing tank I8 through the conduit Il.Sulphuric acid is added to and mixed with the sludge in the mixing tankin an amount suliicient, and preferably not substantially in excess ofthat required, to form ferrie sulphate with all the iron contained inthe sludge. Preferably, the amount of sulphuric acid added to thesludgeis just suilicient to establish in the sludge iron and the sulphate(SOf.)

radical in the proportion in which they would be present in an amount offerric sulphate containing an amount of iron equal to that contained inthe s1udge....The acid-treated sludge is stored in a.

sludge storage tank I9 for subsequent use. A. conduit 20 having a pump2l included therein is provided for conducting acid-treated sludge fromthe storage tank to the reagent supply system. The sludge storage tankis provided with an air inlet to permit the introduction of air formaintaining the acid-treated sludge in an oxidized condition.

The untreated sludge returned to the aeration vessel (20% to 75%, moreor less) may be returned entirely either from the clarifier or from thethickener, or a portion may be returned from a the clariiler and aportion from the thickener.

Both the dryer and incinerator are rotatably mounted cylinders in whichthe solid material and gases (for drying and incinerating) travelconcurrently. Thus, the exhaust gas from the incinerator moves throughthe dryer concurrently with the sludge, and the dried sludge enters theincinerator along with hot gases from an oil burner or other heatingmeans. An adequate amount of oxygen or other combustion supporting gasenters the incinerator (along with the hot gases of combustion of theextraneous fuel) to burn the organic and other combustible matter in thedried sludge. The incinerated residue or ash is discharged from theincinerator to a storage-container from whence it is appropriately fedto a jacketed leaching kettle. In the kettle, the incinerated residue istreated with sulphuric acid, being appropriately agitated and alsoheated if necessary, for the regeneration of ferric sulphate. Thecontents of the kettle is delivered to a centrifuge for the removal ofexcess acid. The centrifuged cake is leached with water to dissolve theferrie sulphate present and the insoluble residue is ltered oiI anddiscarded. The regenerated ferrie sulphate solution is pumped back forreuse in the coagulation of fresh incoming sewage.

It will of course beunderstood that the apparatus illustrated in thedrawing is merely illustrative and is not intended to be restrictive ofthe invention in any respect.

In the following table are given the daily results obtained in testsconducted for the purpose of illustrating the results obtainable bymeans of the process of the present invention. In carrying out the testsof the process of the invention, about 75% of the sludge `obtained inthe clarifier (containing about 4% solids) was returned directly fromthe clarifier to the aeration vessel, aeration was continued for onehour periods and ferric sulphate and acid-treated sludge were added tothe aeration vessel about ve minutes before the end of the aerationperiod. The sewage was treated alternately with an amount of ferriesulphate equivalent to 20 parts per million of iron and one-quarter ofthe sludge treated with 68 p. p. m. 60 B. sulphuric acid, three-quartersof the sludge being returned to the aeration vessel continuously. Theacid-treated sludge was added at .such a rate as to introduce iron inamount equivalent to about 20 parts per million. The biochemical oxygendemand (B. O. D.) and suspended matter are given in' parts per million.The puried eilluent was the supernatant liquor from the clarier. Thestandard day test, using seeded diluting water, was employed indetermining the biochemical oxygen demand.

Raw Emnent Percent com- Date liied r uc- B. O. D Susp. v B. 0. D. Susp.tion 135 172 16 13 91 138 180 13 13 92 163 320 15 9 94 105 112 6 16 9012o 188 10 8 94 183 180 30 20 87 143 176 15 1l 92 148 240- 16 12 92 98112 16 9 90 158 180 18 18 v 90 148 100 18 8 92l 133 148 t 22 18 86 173224 8 10 95 143 128 17 12 89 Total.; 1988 2580 4220 117 Average. 142 18416 13 91 While I have described my preferred process as including theuse of ferric sulphate in conjunction with acid-treated sludge, it willbe understood that other metal compounds may be used advantageously inaccordance with my invention. Thus, for

able. using ferrie sulphate may be obtained by employexample, ferroussulphate, when added at an ing aluminum sulphate. .Mixtures of various.compounds may be employed advantageously, and

desirable results may be obtained by adding two or more compoundsseparately. For example, I have obtained extremely good results byadding small amounts of aluminum sulphate within a .minute or two afterthe nal addition of ferrie sulphate.

The amounts -of aluminum sulphate employed" were equal to abbut tenpercent ot the amounts of ferrie sulphate-employed. I have also obtainedvery desirable results by adding a solution made by dissolving bothaluminum sulphate and ferric sulphate, about 1 part of aluminum to 10parts of iron.

, The process oi! my invention is admirably adaptedfor the treatment ofindustrial waste liquors as Well as ordinary municipal or domesticsewages. It is characteristic of the process of the invention that anextremely rapidly settling sludge is produced and-.high elimination ofputrescible matter is accomplished. Sludge produced in carrying out themethod of the invention settles several times as fast as sludge producedin processes in common use. 4It is not unusual to obtain upon standingfor live minutes a clear supernatant liquid which may be decanted, and Ihave accomplished B. O. D. reductions ofmore than 96% in the normaloperation of the process on manyoccasions. y

I claim: 1. The improvement in the treatment of sewage containingputrescible matter which comprises subjecting the sewage to the actionof a watersoluble compound of a trivalentmetal to coagulate theputrescible matter, separating the coagulated putrescible matter in theform of a sludge from the accompanying liquor, Asubjecting the sludge,

without drying, to the action of an acid capable of regenerating and inamount not substantially in excess of that required to regenerate thesoluble metal compound, maintaining oxidizing condition Within the bodyof the acid-treated sludge and utilizing the acid-treated sludge inconjunction with added trivalent metal compound for the treatment ofadditional sewage by incorporating said acid-treated sludge in thesewage to coagulate putrescible matter therein'.

2. In a process for treating sewage to eiect the removal of putresciblematter involving aeration of the sewage in the presence of returnedsludge and a water-soluble compound of a metal of the group consistingof iron, aluminum and manganese to eiect coagulation of the putresciblematter, and recovery of a sludge containing the coagulated putresciblematter and the metal of the soluble metal compound employed to effectcoagulation, the improvement which comprises returning a portion of therecovered sludge directly tothe sewage undergoing treatment, treatinganother portion or the recovered sludge, without drying, with an acidcapable of forming and in amount'not substantially in excess of thatrequired to form av water-soluble compound by reaction with said metalcontained in the sludge, maintaining oxidizing condition in,l the bodyof the acid-treated sludge from the time otitsproduction untilthe vtimeof its use, and utilizing the acid-treated sludge in commotion with thesoluble metal compound to eiiect coagulation ofV the putrescible matterlby incorporating saidacid-.treated sludg in the sewage undergoingtreatment,

JOHN BEVAN.

